Consider a light source that is constructed from three component light sources in which each component light source emits light in a different region of the optical spectrum. The light from this source will be perceived by a human observer to be of a single color that is determined by the intensities of the component light sources and the spectrum of light emitted by each component light source. If the component light sources emit light in sufficiently different regions of the optical spectrum, the perceived color can be varied over a large gamut of colors. Light sources that utilize red, green, and blue (RGB) component light sources are commonly used to generate light in this manner.
The perceived colors that can be generated with such a light source are often represented in a two-dimensional color diagram in which the component light sources define a triangular area that contains the various color points that can be reached by that light source. In general, not all of the possible color points in the color space can be reached with a three-component color light source that utilizes existing light sources such as light emitting diodes (LEDs). To reach color points in part of the remaining area in the color space, a fourth light source is often utilized. The fourth light source expands the available color points to those bounded by a quadrangle that contains the original triangular area.
When the color point is determined by three light sources, there is a unique solution to the problem of adjusting the intensities of the three light sources to provide a particular color point within the available color space. However, when 4 light sources are used, in general, there is no longer a unique solution to the problem of determining the intensities of the 4 light sources that will provide a particular color point. That is, there are a number of different combinations of light source intensities that will produce the same perceived color. Hence, a different control strategy is required to decide on the combination of intensities to use.
If the output of each of the component light sources maintains a constant as a function of the power applied to that component light source over time, the power levels can be set by an appropriate algorithm and the light source will maintain the desired color. However, if the spectrum from one or more of the component light sources changes over time, the situation becomes more complicated. Light sources that utilize LEDs often fall in this category.
LEDs have a number of advantages over conventional light sources based on incandescent lamps or fluorescent tubes, and hence, there has been considerable interest in color light sources based on LEDs. LEDs have significantly longer lifetimes than both incandescent lamps and fluorescent tubes. In addition, LEDs already have significantly greater efficiency in converting electrical power to light than incandescent light bulbs. In fact, LEDs in some color ranges already have higher conversion efficiencies than fluorescent tubes. Finally, LEDs do not require the high voltages associated with fluorescent tubes. Hence, LEDs are an attractive alternative for such variable color light sources.
Unfortunately, LEDs suffer from aging effects. The light output for a given current through the LED tends to decrease over time. The rate of aging depends on the specific type of LED and can also vary from production lot to production lot. Hence, the component light sources age at different rates. As a result, the perceived color of the light source shifts with time if corrective action is not taken.
To correct for aging problems, LED-based light sources often include a photodetector that monitors the output of the light source in a number of wavelength bands and a feedback system that varies the current through the LEDs as the LEDs age to assure that the output of the light source remains at the same point in the color space as the light source ages. Typically, the light from the LEDs is monitored in three different color bands that approximate the red, blue, and green emission bands of the corresponding LEDs. The photodetector typically consists of 3 photodiodes. Each photodiode is covered by an appropriate bandpass filter that assures that the photodiode output is related to the intensity of light in the corresponding band.
The output of the photodiodes does not exactly correspond to the emission spectra of the LEDs. As a result, the signal from any given photodiode typically represents a weighted sum of the light output of more than one LED. To provide values that correspond to the light output of each LED, the signals from the three photodiodes must be processed mathematically to provide a set of corrected signals in which each corrected signal measures the light output from one of the LEDs. The processing typically involves solving a 3×3 system of linear equations.
Given a perceived color that is to be maintained and knowledge of the emission spectra from each of the LEDs, the average light output that is to be provided through each of the LEDs can be determined. This light output is converted to an average current to be provided to each LED. As an LED ages, the measured light output from that LED decreases. The decrease is measured with the photodetector, and the current to that LED is increased until the measured light output is returned to the desired value. Single chip integrated circuit controllers for performing this servo loop are available. These controllers include the hardware to solve the system of the linear equations discussed above.
When a fourth LED is added to the light source to expand the color gamut of the light source, the control strategy discussed above becomes significantly more complicated. In principle, a similar servo control loop can be utilized for such four LED light sources. However, the analogous control system requires a photodetector that provides four color intensity signals in four different spectral bands. The computational workload imposed by the resultant 4×4 system of linear equations is significantly greater than that imposed by the 3×3 system of equations, which increases the cost of the controller. Furthermore, a different controller chip would need to be developed and produced to implement the servo loop for such 4 LED light sources. Since such light sources have significantly smaller markets, the economies of scale available in the present three LED controllers are not yet available.
Furthermore, there is no unique solution to the control problem. For example, if one of the 4 LEDs ages, one could alter the intensity of the LED in question. Alternatively, the intensities of the other LEDs could be altered to provide the same color point and intensity.